An 18-year old Alabama choir girl who just wanted to make friends and have some fun at college has unwittingly uncovered a dark remnant of southern segregation - a clandestine and powerful network of all-white fraternities and sororities known simply as "the Machine".

Melody Twilley is an outgoing and highly intelligent student at the University of Alabama. She attended a prestigious secondary school, earned excellent grades and sang in the university choir. She is also black, which may explain why she was turned down by all the campus's 15 white sororities - the women's social clubs that can guarantee social and professional success at university and far beyond.

The escalating row over the blatant snub, at the same university where, in 1963, the Alabama governor, George Wallace, promised "segregation forever", has cast doubt on just how far the "New South" has gone towards eliminating racism.

According to university insiders, Ms Twilley's blanket rejection was orchestrated by "the Machine", a group of white students and alumni which acts like a shadow government on campus. Machine-backed candidates have won 14 of the past 15 elections for president of the student body.

Liberal candidates who have tried to defeat the Machine's power have been discouraged by threats and worse.

In 1986, when the first black sorority, Alpha Kappa Alpha, was set up, a cross was burned on the lawn outside its building. When Minda Riley, an independent, stood for student president in 1993, she was attacked at night by a knife-wielding masked man. A cross was burned in her garden. and she was sent a note reminding her: "Machine rules, bitch."

The Machine avoids violence these days, but its power clearly remains pervasive on campus and beyond the university gates, where it acts as a professional network for career advancement.

The Machine's members are never named and it holds its meetings in carefully guarded secrecy. When Esquire magazine revealed that the sessions were held in an old gravel pit a few years ago, a new site was chosen, and has not been revealed.

Unaware of what she was up against, and undeterred by a string of earlier rejections, Ms Twilley put on a summer dress this weekend and took part in the "sorority rush", an event marking the beginning of the new academic year, in which female students go from one sorority house to another, trying to make impression with their personality and style, and win a coveted invitation to join.

"I'm not trying to be the next Rosa Parks," she told the Los Angeles Times, referring to the black woman who struck a blow against segregation in 1955 by refusing to sit in the part of the bus reserved for blacks. "I'm just rushing because I think it would be fun to be a sorority girl. I like the idea of sisterhood." Sororities and fraternities, known as the Greek system because each club is identified by Greek letters, are an integral part of US university life. The privileges of membership include a sense of belonging, parties and plenty of introductions to the opposite sex. They can be a passport to social success at university, and to networks of professional contacts afterwards.

Greer Gray, the head of Alabama's Panhellenic Association, which oversees the university's sororities, denied the allegation of racial bias.

"All of the houses give everyone a fair chance," Ms Gray told the Tuscaloosa News. "They're looking for someone who was involved in high school activities and has good grades."

Until this weekend, it was thought that none of the University of Alabama's white sororities or fraternities had ever accepted a black member. There are separate clubs for blacks on campus, which also accept white students.

However, when a public row broke out about Ms Twilley's treatment, another student came forward to say she had crossed the colour bar last year without anyone noticing. Christina Houston, who has a black father and a white mother, said she had been accepted by the all-white sorority, Gamma Phi Beta.

But it appears that Ms Houston was assumed to be white. When she went to parties, she said, she would hear racial epithets flow freely. "Lots of times they would forget what they were talking about, and n-words would go flying like bullets," she said.

Stan Uncaphur, a first-year student who was accepted by a white fraternity but left a few months later in disgust, said the members regularly played games in which they mimicked blacks, and forced candidate members to impersonate black women fighting. The university's faculty senate last month urged the white fraternities and sororities to accept blacks or face penalties. The academic staff pointed out that the university has a significant responsibility for the Greek system as it leases land and buildings to the clubs for token rents.

Ms Twilley is trying to decide whether to continue her attempts to join a white sorority or join one of the black sororities.

"It took the Alabama National Guard to integrate the university," she said. "I hope it doesn't take something that drastic to integrate the Greek system."